As the size of functional elements in integrated circuits decreases, complexity and inter-connectivity increases. To accommodate the growing demand of interconnections in modem integrated circuits, on-chip interconnections have been developed. Such interconnections generally consist of multiple layers of metallic conductor lines embedded in low dielectric constant material, and the dielectric constant in such materials has an important influence on the performance of an integrated circuit. Materials having low dielectric constants (i.e., below 2.2) are desirable because they typically allow faster signal velocity and shorter cycle times. Moreover, lowering of the dielectric constant reduces capacitive effects, leading often to less cross talk between conductor lines and lower voltages to drive integrated circuits.
One way of achieving low dielectric constants in insulator materials is to select materials with inherently low dielectric constants. Generally, two different classes of low dielectric constant materials have been employed in recent years--inorganic oxides and organic polymers. Inorganic oxides often have dielectric constants between 3 and 4, and have been widely used in interconnects with design rules larger than 0.25 .mu.m. However, as the dimensions of interconnects shrink, materials with lower dielectric constants are generally required. Organic polymers have shown many advantageous properties including high thermal stability, ease of processing, low stress/TCE (thermal co-efficient of expansion), low dielectric constant and high resistance. Organic polymers are therefore frequently considered as alternative low dielectric constant polymers for the 0.18 .mu.m and 0.13 .mu.m generations.
Another way of achieving low dielectric constants is to introduce air into an appropriate material, since air has a dielectric constant of about 1.0. Air is usually introduced into a material by formation of minute voids (also referred to herein as pores), with a size in the sub-micrometer range. Such porous materials are then usually termed "nanoporous materials".
It is known to produce nanoporous polymers by providing a polymer with thermolabile groups, and then thermolyzing the thermolabile groups to produce voids. In prior art FIG. 1, for example, a monomer 1 is provided comprising a monomer backbone portion M having a thermo-labile group L. A polymer 2 is formed by polymerizing n repeating monomer 1, and the polymer 2 is subsequently crosslinked to form a crosslinked polymer 3. In a further step, the crosslinked polymer 3 is thermolyzed to remove at least some of the thermolabile groups L, thereby producing a nanoporous polymer 4 containing voids V. The method is conceptually simple, but typically allows only poor control over pore size and pore distribution.
In another approach, a thermostable polymer is blended with a thermolabile polymer. The blended mixture is then crosslinked and the thermolabile group thermolyzed. Examples are set forth in U.S. Pat. No. 5,776,990 to Hedrick et al. An advantage of this approach is that variations and modifications in the thermolabile polymer and the thermostable polymer are readily achieved. However, blending thermolabile and thermostable polymers once again typically allows only poor control over pore size and pore distribution.
In still another approach, thermolabile blocks and thermostable blocks alternate in a single copolymer, often termed a block copolymer. In prior art FIG. 2, for example, monomers A, B, C, and D, collectively identified with numeral 5, are provided in which at least one of the monomers carries a thermolabile group L, and at least one of the monomer carries a crosslinker. The monomers A, B, C, D are polymerized to form block oligomers 6, and n repeats of the block oligomers 6 are further polymerized to form a block copolymer 7. The block copolymer 7 is subsequently crosslinked to form a crosslinked block copolymer 8, and the thermolabile group is thermolyzed, resulting in a nanoporous polymer 9. This approach is advantageous in generally allowing good control over pore size and pore distribution, but may decrease the ultimate thermal and dimensional stability of the nanoporous material due to fragmentation of the polymer upon thermolysis of the thermolabile group. In addition, if the type or length of the thermolabile group need to be changed, new blocks and new copolymers must be synthesized.
Thus, in the known methods of introducing voids into nanoporous materials by thermolyzing thermolabile groups of polymers, there is an unfortunate tradeoff between control over pore size and pore distribution, and simplicity and flexibility of design and synthesis. By gaining control over the nature of the repeating units carrying the thermolabile group(s), one substantially dictates the qualities of the resulting nanoporous polymers. Modifications to the amount, chemical nature and positioning of the thermolabile group are not feasible once polymerization is finished.
Regardless of the approach used to introduce the voids, structural problems are frequently encountered in fabricating nanoporous materials. Among other things, increasing the porosity beyond a critical extent (generally about 30% in the known nanoporous materials) tends to cause the porous materials to collapse. Collapse can be prevented to some degree by adding crosslinking additives that couple thermostable portions with other thermostable portions, thereby producing a more rigid network. U.S. Pat. No. 5,710,187 to Streckle, Jr., describes crosslinking for this purpose, crosslinking aromatic monomers using multifunctional acyl- or benzylic halides.
Crosslinking of thermostable portions in nanoporous materials frequently has significant limitations. For example, the crosslinking agent needs to specifically react with the thermostable portion without interfering with the polymerization reaction. Moreover, the chemical structure of the thermostable portion and availability of reactive groups dictates the nature of the crosslinking agent. In addition, the crosslinking agent must be soluble in the same solvent system as the block copolymers or monomers. Still further, new and potentially useful additional moieties must be compatible not only with the chemical properties of the block copolymers, but also should not interfere with the polymerization reaction. In general, the introduction of functional elements into nanoporous materials by copolymerization is limited to the synthesis and availability of the block copolymers or monomers.
In summary, many methods are known to improve the physicochemical properties of nanoporous materials. However, current methods tend to limit the ease with which differing functional elements can be incorporated. Surprisingly, despite great efforts to improve various properties in nanoporous materials, and many efforts to modify individual components of nanoporous materials, there is no system that permits relatively simple modifications of precursors to produce desired properties in the end products. Therefore, there is still a need for methods and compositions that permit relatively simple modifications of the precursors to produce desired properties in nanoporois materials.